In the field of semiconductor manufacturing, it has been recognized since the beginning of the industry that removing particles from semiconductor wafers during the manufacturing process is a critical requirement to producing quality profitable wafers. While many different systems and methods have been developed over the years to remove particles from semiconductor wafers, many of these systems and methods are undesirable because they cause damage to the wafers. Thus, the removal of particles from wafers must be balanced against the amount of damage caused to the wafers by the cleaning method and/or system. It is therefore desirable for a cleaning method or system to be able to break particles free from the delicate semiconductor wafer without resulting in damage to the device structure.
Existing techniques for freeing the particles from the surface of a semiconductor wafer utilize a combination of chemical and mechanical processes. One typical cleaning chemistry used in the art is standard clean 1 (“SC1”), which is a mixture of ammonium hydroxide, hydrogen peroxide, and water. SC1 oxidizes and etches the surface of the wafer. This etching process, known as undercutting, reduces the physical contact area to which the particle binds to the surface, thus facilitating removal. However, a mechanical process is still required to actually remove the particle from the wafer surface.
For larger particles and for larger devices, scrubbers have been used to physically brush the particle off the surface of the wafer. However, as device sizes shrank in size, scrubbers and other forms of physical cleaners became inadequate because their physical contact with the wafers cause catastrophic damage to smaller devices.
The application of acoustic energy during wet processing has gained widespread acceptance to effectuate particle removal, especially to clean sub-micron particles off wafers (or plates) undergoing fabrication in the semiconductor process line. The acoustic energy used in substrate processing is generated via a source of acoustic energy. Typically, this source of sonic energy comprises a transducer which is made of piezoelectric material, such as a ceramic or crystal. In operation, the transducer is coupled to a source of electrical energy. An electrical energy signal (i.e. electricity) is supplied to the transducer. The transducer converts this electrical energy signal into vibrational mechanical energy (i.e. acoustic energy) which is then transmitted to the substrates being processed. The transmission of the acoustic energy from the transducer to the substrates is typically accomplished by a fluid that acoustically couples the transducer to the substrate. It is also typical that a material capable of acoustic energy transmission be positioned between the transducer itself and the fluid coupling layer to avoid “shorting” of the electrical contacts on the piezoelectric material. This transmitting material can take on a wide variety of structural arrangements, including a thin layer, a rigid plate, a rod-like probe, a lens, etc. The transmitting material is usually produced of a material that is inert with respect to the fluid coupling layer to avoid contamination of the substrate.
The application of acoustic energy to substrates has proven to be a very effective way to remove particles and to improve the efficiency of other process steps, but as with any mechanical process, damage to the substrates and devices thereon is still possible. Thus, acoustic cleaning of substrates is faced with the same damage issues as traditional physical cleaning.
The acoustic energy generated by existing transducer assemblies is often energetic enough to cause some of the fragile structures that make up the electrical circuit to be damaged (i.e., removed or partially removed causing the circuit to no longer function). Through long-term study of existing transducer assemblies and the associated acoustic properties, the current inventors have determined that a myriad of problems exist both with the structure of the piezoelectric material and the direction and orientation of the acoustic waves propagated by existing transducer assemblies.